Electrical identification circuit



Sept. 27, 1960 s. SIMONv ET AL ELECTRICAL IDENTIFICATION CIRCUIT FiledFeb. 4, 1957 8 Sheets-Sheet 1 J. ROSENOER A Harney 4 Sept. 27, 19602,954,439

S. SIMON ETAL ELECTRICAL IDENTIFICATION CIRCUIT Filed Feb. 4, 1957- 8Sheets-Sheet 2 L l 5., f (5)0274) p'XX (p-f) Y2? l y: I y/ *We g i/QK/ff-/xfy/ fg 1 3|; /V/*PYQ Wh/) T (@/m--fmw? i L rfi/f2 FIG .7.

Inventor S'SADNNOR ByJR gw A ttarney Sept. 27, 1960 s. SIMON ErAL2,954,439

ELECTRICAL IDENTIFICATION CIRCUIT A Harney Sept, 27, 1960 s. slMoN ETAL2,954,439

ELECTRICAL IDENTIFICATION CIRCUIT Filed Feb. 4, 1957 a sheets-sheet 4 nentor s. SIMUON JROSENOER By www Attorney Sept. 27, 1960 s. SIMON ET'AL2,954,439

ELECTRICAL IDENTIFICATION CIRCUIT Filed Feb. 4, 1957 8 Sheets-Sheet 5Inventor S. Si M O N J. ROSEN@ R By M Sept. 27, 1960 s.s1MoN ETAL2,954,439

ELECTRICAL IDENTIFICATION CIRCUIT Filed Feb. 4, 1957 e sheets-sheet e El5]^/\I m2 4m I (m4) 2/ Dl?) I E 64m) Attorney Sept. 27, 1960 s. SIMONETAL 2,954,439

ELECTRICAL IDENTIFICATION CIRCUIT A Harney Sept. 27, 1960 s. sxMoN ETAL2,954,439

ELECTRICAL. IDENTIFICATION CIRCUIT File Feb. 4, 1957 8 Sheets-Sheet 8 lC5 )'*0 E BU/V\1| E L -J F L .i i @l x y xy XC C FIG .53. 5 5

SSIMON J. ROSENER By.

Attorney assignors ,to International Standard Electric Corporation, NewYork, NX., a corporation of Delaware Filed Feb. `4, 1957, Ser. No.638,089

Claims priority, application Belgium Feb. 10, 1956 l9 Claims. (Cl.179-18) The invention relates to fan electrical identification circuit.

More particularly, the invention relates to -an electricalidentification circuit including a plurality of input .circuits each ofwhich is connected to a combination of output circuits characterisingthe input circuit, by means of a network of passive elements constitutedby impedances. By means of such Ia network, when an A C. energy sourceis applied to one of the said input circuits, energy will be transmittedto each of the circuits whose combination characterises the inputcircuit to which the A.C. energy source is applied. The output circuitswill each include a detecting device in order to react to theapplication of the A.C. energy which reaches it, and to operate anybistable device -such as a relay in order to cause a registration of theidentity of the input circuit to which the energy source has beenapplied, by the simultaneous operation of a relay combinationcorresponding to the activated input circuit.

An identification circuit of the above mentioned type can beadvantageously used in ra telephone exchange in order to be able toidentify a calling line. Such an 4identification will in general'berequired for ticketing purposes. The circuit wishing to obtain theidentification of a calling line will cause the application of an A.C.energy source to a test conductor of the calling line by means of theswitches and of their circuits which are already branched on the callingline. An A C. potential will thus be brought by the test conductor tothe line circuit of the calling line. The line circui-t constitutes oneof the input circuits of the identifier, and consequently the A.C.energy will cause the operation ofv a relay combination which willcharacterise the number of the calling line, and one will be able totransmit its identity tothe circuit having requested the identificationof the calling line.

` there exist four groups of ten.

From then on, the identification circuit has finished its n function`and can be usedv again to determine the identity of -another callingli-ne.

When the identification circuit is essentially constituted by animpedance network, it possesses the advantage that the identication ispractically instantaneous and depends only upon the operating time ofthe relays included in the output circuits. Thus, it is seenthat thetime necessary to identify a calling line can be very short and of theorder of a few hundreds of' seconds. Consequently, the number ofidentification circuits `for a telephone exchange can be Vextremelysmall since an identifier is used only during `an extremely short time.To the time necessary to operate the combination of relayscharacterising the calling line, one must of cou-rse add the seizuretime of the identification circuit and its release time, in order to beable to determine the required Vnumber of identification circuits infunction of the telephone traic. In any ease, it is perfectlyconceivable that only one identification circuit of the type mentionedabove need bel provided in a telephone exchange.

An identification circuit of the typediscussed labove aesinet PatentedSept, 27, i930 .substantially 4distinguishes itself from identificationcircuits used until noW in telephone exchanges. Until now, the circuitsused generally `accomplish the identification function in a sequentialmanner. An electrical signal is applied to a test wire of the callingline which it is desired to identify, but the detection of this signalby the identification circuit (and which must cause the operation ofrelays or equivalent devices in order to characterise the calling lineto which the signal is applied), is not directly made.` Hunting takesplace by means, for example, of telephone finders in order to determineto which line the signal has been applied.

'For more details concerning these identification systems 4alreadyapplied, one will be able for instance to usefully refer to the U.S.Patent No. 2,338,242. With- "out entering into the kdetails of sucharrangements, it can ybe said that the hunting time which is necessaryfor the identification, before one can obtain the identity of a callingline, is essentially due to the necessity of avoiding parasitic.couplings between the Various lines and which can result in theapplication of part of the energy provided to the line which must beidentified, to detecting devices which do not correspond to thedetecting devices whose combination characterisesthe line to which thesignal is applied.

A sound general View of the identification problem and in` particularthe identification in telephone exchanges, may be obtained -by referringto the article of H. H. Schneckloth which appeared in the July, 1951issue of The Bell System Technical Journal on page 588 and' further, andentitled Some basic concepts of translators and identifiers used intelephone switching system. In particular, this article brieflydescribes a system using an A.C. energy source to identify a callingline. It is to be remarked that the use of an A.C. energy sourceprovides the advantage that it can be applied by means of a condenser totest conductor which remains free for DLC. lcurrent signalling. If onerefers particularly to Fig. l3(a) of the above mentioned article, itwill be seen that each sleeve conductor of the line is permanentlyconnected to ground by means of a condenser in series witha resista-nce.The junction point of the condenser and of the resistance, and which isnot connected -to ground, is connected by means of four resist- .ancesindividual to each line to .detecting devices of which The firstresistance of each line is connected to one of the detecting devices ofthe arst group, the second resistance is connected to one of thedetecting devices of the second group, and so on. In this manner, whenan A.C. current is sent in the sleeve conductor of the calling line tobe identified, a portion of this current will reach four detectingdevices which will operate andA the combination of which willcharacterise the calling line number on afour-digit decimal basis. Thecondenser connected .to cach sleeve conductor prevent anyvD.C. couplingbetween various sleeve conductors belonging to different lines. Suchcouplings must be avoided' in order that the D.C. signals circulating ona sleeve conductorV should not reach other sleeve conductors. ThegroundA connected resistance limits the backup reffects which have untilnow been unavoidably associated with identification circuits of thistype. The backup .coupling .can -be defined by the followingexplanation:

Assuming'that various lines arev connected to av same detecting deviceby means of yone of their four Vresistances, the signal portion.reaching a detecting device corresponding .to the line to .be identifiedwill also be transmitted -to these other lines; iFrom there on, aportion `of this energy Will be applied to detectors which do notcorrespond tothe four, detectors to which the line to be identified isdirectly connected by means of these four A resistances.

The identification system briefly described above and of which theessential element is an impedance network to perform the coding of theidentity of a calling line towards a limited number of detectors, isdescribed in more detail in the U.S. Patent No. 2,672,515. In order toreduce the back-up couplings to a value such that the discriminationbetween signals which must lactivate detectors and those which must notactivate them, can be easily performed, a very low value for theresistance individual to each line which is connected t ground isfavoured. On the other hand, the series rcsistances going towards thedetectors will be substantially high. The respective values mentionedare 100 and `10,000 ohms. Although this system effectively permits agood discrimination be- 'tween the effective signals yand the parasiticsignals, this is nevertheless obtained at the expense of a considerableenergy loss for the effective signals with respect to the availablesource energy. Indeed, when there is a large number of lines to beidentified, such 'as 10,000, an appreciable number of series resistancesare effectively placed in parallel towards ground in so far as thesignal transmission towards the `detectors which must be activated isconcerned. For a 10,000 line system wherein the ratio between the seriesresistance and the resistance towards ground is 100, one will obtain adiscrimination of the order of 40 decibels between the effective signalsand the parasitic signals, but this will unavoidably entail a loss ofthe order of 60 decibels between the energy available from the sourceand the said effective signals. It results therefrom that each detectormust be provided with means for considerably amplifying the receivedsignal, in order that the reception of this signal shall ultimatelycause the operation of a telephone-type relay. As shown in the patent,three amplifier tubes are necessary in order to amplify the signalreceived by each detector, and in addition to the associated circuits,one must still provide filters and an integrating device, in addition tothe rectifying device which will provide a D.C. voltage capable ofoperating the relay. Additional electrical networks will also berequired in order to check the accuracy of the received code. Indeed,due to the considerable transmission loss, the signals which reach theinputs of the detectors are at a very low level, and consequently aresubject to be iniiuenced by any spurious signals. This low level alsoappears as being the reason motivating the use of filters andintegrating devices which will insure a certain degree of immunity fromspurious signals. The verification of the received code will consist inensuring that the number of received signals for each group of detectorsis constant. ln fact, the coding network permits to identify on thebasis of four groups of five detectors, a combination of two vdetectorsout'of five being operated Within each group. In order to avoid the useof eight series resistances per line permitting a coupling towards eachof the eight detectors corresponding to the line, the coupling isperformed in two stages. A first stage connects each line to twointermediate points of which each is taken within a group of 100 points.lIn turn, each of the intermediate points is connected by means of fourseries resistances to four detectors, two out of a first group of fiveand two out of a second group of five, the two first groups of fivedetectors corresponding to the first group of 100 intermediate pointsland the two last groups of five detectors corresponding to the secondgroup of 100 intermediate points.

On the whole, it is seen that the loss of useful energy is responsiblefor making the detecting circuit rather complicated.

As object of the invention is to provide an identifier of the codingnetwork type comprising passive'elements, and arranged in such a waythat there is no strict relationship between the transmission of theenergy towards the detecting circuits which must be activated tocharacterise a line to which this energy is applied; and the differencein level between the signals lapplied to the detectors which and thosewhich must not be activated is substantially nil.

-Another object of the invention consists in providing .an identifier ofthe above type and for which the A.C. source provides substantially amaximum of energy, this by impedance matching, the whole of thisavailable energy being substantially divided among the detectors to beactivated.

In accordance with a characteristic of the invention, an electricalidentifier including a plurality of input circuits, a plurality ofoutput circuits, a coding network comprising a plurality of couplingimpedances between each input circuit and a combination of outputcircuits characterising the input circuit, whereby an A.C. energy sourcemay be applied to an input circuit causing thereby the delivery ofenergy to the output circuits forming the said combination correspondingto the activated input circuit, is characterised by additional couplingimpedances provided in order that the equivalent transfer impedancebetween any pair of output circuits of which one is part of acombination of output circuits characterising an input circuit to whichenergy is applied, and' of which the other is not part of the saidcombination, is substantially infinite at the frequency of the A.C.energy source, whereby an arrangement without back-up is obtained, noenergy reaching those output circuits which are not part of thecombination of output circuits characterising the input circuit to whichA.C. energy is applied.

.In accordance with another characteristic of the invention, anelectrical identifier as defined above, is characterised by the factthat the said coupling impedances and the said additional couplingimpedances are purely reactive, whereby it results that no energy isdissipated in the coupling networks and that the whole of the energyavailable from the A.C. source applied to an input circuit can bedistributed among the corresponding combination of output circuits.

In accordance with yet another characteristic of the invention, anidentifier such as defined above is characterised by the fact that theoutput circuits include rectifiers which serve not only to rectify theA.C. energy reaching an output circuit, but also to form a final codingnetwork destined to reduce the number of detecting devices such astelephone relays, the rectifiers also permitting an effective decouplingbetween any pair of detecting devices comprising an activated detectingdevice and another.

'I'he objects and characteristics of the invention mentioned above aswell as others, will become more apparent by referirng to the followingdetailed description to be read in conjunction with the accompanyingdrawings and where:

Fig. 1 schematically represents a first embodiment of the invention;

Fig. 2 represents a first equivalent circuit useful to explain thecircuit of Fig. 1;

Fig. 3 represents a second equivalent circuit useful to explain theoperation of the circuit of Fig. l;

Fig. 4 represents the circuit equivalent to that of Fig. l when the A.C.energy applied to an input circuit is entirely divided between theoutput circuits whose combination characterises this input circuit;

Fig. 5 schematically represents a second embodiment of the invention;

Fig. 6 represents the circuit equivalent to that of Fig. 5 when an A.C.energy source is applied-to one of the input circuits, but solely in sofar as the coding network conllecfng the input circuits to the outputcircuits is concerned;

Fig. 7 represents a new circuit substantially equivalent to the circuitof Fig. 6; t

Fig. 8 represents yet another circuit substantially .equivalent to thoseof Figs. 6 and 7;

Fig. 9 represents a network equivalent to .that of Fig. 5 when an A C.energy source is applied to one of the input circuits, but correspondingsolely to the additional coupling impedances constituting the'back-upneutralising network;

Fig. 10 represents a circuit rigorously substantially similar to that ofFig. 9;

Fig. 11 represents the equivalent circuit corresponding to the whole ofthe network of Fig. 5 and obtained by the superposition of theequivalent circuits .of Figs. 8 and 10, and when the elements of thenetworks are chosen so that the whole of the energy provided from thesource is divided solely between the output circuits forming thecombination corresponding to the input circuit which `is activated;

Fig. 12 schematically represents a third embodiment of the invention;

Fig. 13 represents the equivalent circuit of the network of Fig. 12 whenan A.C. energy source is applied to one of the input circuits, butsolely in so far as the coding network connecting the input circuits tothe output circuits is concerned;

Fig. 14 represents the equivalent circuit of the network of Fig. 12 whenan A.C. energy source is applied to one of the input circuits, butsolely in so far as the back-up neutralising network is concerned;

Fig. 15 represents the equivalent circuit for the whole of the networkof Fig. l2 and corresponding to the superposition of the equivalentnetworks of Figs. 13 and 14, when the whole of the energy applied to oneof the input circuits is entirely divided between the output circuitwhose combination characterises the activated input circuit.

Fig. 16 represents a modification of the back-up neutralising networkshown in Fig. l2;

Fig. 17 represents the equivalent circuit of the neutralising network ofFig. 16 when an A.C. energy source is applied to one of the inputcircuits;

Fig. 18 schematically represents a fourth embodiment of the inventionwherein the coding network between the input and output circuitscomprises two cascaded coupling stages;

Fig. 19 represents an equivalent circuit of lthe network of Fig. 18 whenan energy source is applied to one of the input circuits, but solely inso far as the coding network between the input and the output circuitsis concerned;

Fig. 2O represents a circuit substantially equivalent of that of Fig.19;

Fig. 2l represents the equivalent circuit of the network of Fig. 18 whenan energy source is applied to one of the input circuits, but solely inso far as a rst part of the back-up neutralising network is concerned;

Fig. 22 represents a circuit substantially equivalent to that of Fig.2l;

Fig. 23 represents the superposition of the equivalent circuits of Figs.2O and 22 when a certain relation between the elements is satisfied;

Fig. 24 represents the equivalent circuit of the network of Fig. 18 whenan energy source is appliedto one of the input circuits, but solely inso far as a second part of the back-up neutralising network isconcerned;

Fig. 25 represents the superposition of the equivalent circuits of Figs.23 and 24 when an additional vrelation between the elements issatisfied, and such that the whole of the energy applied to one of the'input circuits is divided between the output circuits whose combinationcharacterises the activated input circuit;

Fig. 26 schematically represents an embodiment of :a back-upneutralising network and whose elements are purely reactive;

Fig. 2'7 represents the equivalent circuit of the network of Fig. 26when [the latter is activated;

Fig. 28 schematically representsa detailed embodiment of two outputcircui'tswhen two AC. energy sources may be applied to the inputcircuits, the frequencies of these two sources being distinct;

Fig. 29 represents a back-up' neutralising network corresponding to thatof. Fig. 26, but adapted to be effectivev at two distinct frequencies; t

Fig. 30 represents the equivalent crcuitof the network of Fig. 29 whenthe latter is activated;

Fig. 3l represents a circuit substantially equivalent to that of Fig.30;

Fig. 32 represents a back-up neutralising network solely comprisingresistors and condensers;

Fig. 33 represents the equivalent circuit of the network of Fig 32 whenthe- Ilatter is activated.

Before referring to the various figures starting with Fig. 1, it will beuseful to remark that all the embodiments which will be described willshow unbalanced circuits, i.e. all incorporating a ground returncircuit. This arrangement is, of course, not essential and the variousunbalanced network embodiments may all be realized by means of balancednetworks, i.e. with respect to ground.

It should also be noted that the term admittance will generally be usedin the remainingl part of the description, this ldeiinitio-n of theelements being more appropriate to the types of analysis required.

By referringv to Fig. 1, the latter represents an identification circuitpermitting the identification of one point among a plurality of (t) n vpoints by means of m detecting devices. The expression represents thebinomical coecient and is expressed by points A of which any may beidentiiied by the application of an input signal to this particularpoint, one only has been represented. The denomination has however beenmarked below A in order to character.- ise the number of these points.This practice will be repeated for the various figures accompanying thedescription and all the multiple points as well as` the multipleelements will be followed .by an indication as to their number in thecomplete network. From` Figi it is seenV that each A point is connectedto n points B, each time with the help of a dipole having an admittanceY2. As for the A points, only a single B point has been represented, butthere exist m for all of the network. The fact that each A point isconnected to a combination of n points B particular to this point,v isindicated by the multipling arrow marked with n. For the whole of thenetwork there wll thus be '7 admittances Y2. In way, it will be realizedthat each B point is connected to fn): m n

points A. The admittance of each of these A points with respect toground has been indicated by Y1, while the admittance of each of the Bpoints with respect to ground has been represented by Ys.

In addition to what has just been said, Fig. 1 shows a back-upneutralising network BUN which is connected to the various B points.Before considering the effect of this BUN network, the network of Fig. 1will first of all be studied without this BUN network.

The equivalent admittance between any B point and ground will first ofall be considered. As the multipiing arrow near the B point indicates,each of the latter is connected to Y (m-I n 1 points A. Each of thesenetworks which connect the B point considered to this plurality of Apoints, is essentially a star network, each A point being connected toground by the admittance Y1 and to n points B each time through anadmittance Y2. It is generally known that a three-branch star networkcan be transformed into a triangle network by elimination of the starpoint, the triangle network being in all respects equivalent to the starnetwork. On the other hand, in the Journal of the Institution ofElectrical Engineers of 1924, volume 62, page 916, this equivalenceproperty has geen generalised for the case of a transformation of a starnetwork having one star point and any number of branches. The admittancebetween any pair of points, when the star point has been eliminated, isequal to the product of the admittances which connected each of thepoints forming the pair to the star point which one `desired toeliminate, this product being divided by the sum of all the admittancesconnected to the star point.

Consequently, for each admittance star having an A point as centre, onewill have an admittance between a point B part of the star consideredand ground, which will be equal to If a particular pair of B points suchas B and B indicated in Fig. 2 is considered, this pair of B points willbe found Y n-2 star networks issued from a corresponding number of Apoints. Therefrom, `it results that the equivalent admittance betweenany pair of B points is that represented in Fig.' 2.

As there exists an equivalent -admittance between any pair of B points,any application of energy to a particular A point for the purpose ofcollecting energy at the n point B corresponding to the A pointconsidered, will have the result of also applying a certain energy tothe remaining (m-n) points B.

The quantity of parasitic energy appearing at the (m-n) points B whichdo not characterise the A point which one wishes to identify will dependon the equivalent admittance joining each pair of B points.

The BUN network shown in Fig. 1 will permit the reduction of theladmittance between lany pair of B points to zero. Indeed, in accordancewith Fig. l, each of the m points B is connected with the help of theBUN network to ta unique point F, each point by means of a 'dipole whoseadmittance is indicated by Y.1. Thus, there are m admittances Y.1 forthe whole of the network considered. Further, the point F has anadmittance with respect to the ground represented by Y5. As will be seenlater, this admittance Y5 is not in fact essential, and this is alsotrue for the Kadmittances Y1 connecting each A point to ground.Nevertheless, in many networks, such admittances will in fact bepresent, parasitic capacitances to ground for example.

'Ihe theorem of equivalence between a star network and the polygonnetwork will again be used to determine the equivalent admittancebetween any B point and ground, and resulting from the BUN network. Theequivalent admittance between any pair of B points and also resultingfrom the BUN network will be obtained in the same way.

These equivalent admittances have been represented in Fig. 3. It will benoted that neither Fig. 2, nor Fig. 3 represent the admittance Y3 which,of course, connects each B point to ground. The total equivalentadmittance interconnecting any pair of B points is thus the sum of thecorresponding admittances represented in Figs. 2 and 3. Consequently,if'one satisfies the relation (1i-2)Y1+7LY2+mY.+Y.-0 (2) the equivalentadittance between any pair of B points will thus be nil, and no fractionof the input energy applied to a particular A point shall reach the(m-n) B points which are not directly connected by an admittance Y2 tothe A point considered.

If one considers the Relation 2 it is seen that at least one of theadmittances such as Y.1 for example shall have to -be of ya signcontrary to that of the admittances Y1 and Y2. Consequently, if A.C.energy is applied to a particular A point for the purpose of identifyingit, the admittances such as Y1, Y2, Y.1 and Y5 can be realized by meansof elements whose admittance is a function of frequency. In particular,the admittances Y1 and Y2 may each be simply constituted by a'condenser, while on the other hand each of the admittances Y.; and Y5will be constituted by `an inductance. For such cases, it will bepossible to satisfy the Relation 2 at a well determined frequency. Thisamounts to saying that the frequency of theenergy source used to feedthe A point having been determined, the elements of the network will bechosen in function of this frequency in order to satisfy the Relation 2.

The Relation 2 being satisfied, a very simple equivalent network canrepresentthe complete circuit of Fig. l, a source of A.C. voltage ehaving an admittance Y1, is applied to =a particular A point which willbe called An. Only the n points B which are directly connectedv to thisparticular An point must be taken into consideration since the remainingB points, after elimination of all the A points except An bystar-polygon transformations, are completely ldecoupled from the energysource. As one is forced to keep the An point in the circuit since it isat this point that the A.C. voltage source is applied, the n' points Bdirectly connected to this point will be conpoints A which will be equalto Y1 -l- 'n Y2 On the other hand, the equivalent admittance of each ofthese B points with respect to ground will be equal to the sum of theequivalent admi-ttances represented in Figs. 2 and 3 and respectivelyresulting from .theclimination of the A points and of the F point, minusan admittance equal to the equivalent admittance corresponding to the Anpoint which has not been eliminated. ,As each .of these, B points isdirectly connected to the An point by admittances having the same value,i.e. Y2, Iand as on the other hand each of these B points is connectedto ground by an admittance which has the same value vfor all these Bpoints, the latter will all be at an'equal potential. Therefrom, itresults that the various residual admittances which interconnect them inpairs must not be taken into consideration.

Hence, the equivalent circuit ofthe arrangement shown in Fig. 1 in thecase where a source e is applied to a particular An point, can berepresented by Fig. 4 where pf-irt B1 is on the one hand connected topoint An tlrsugh an admittance nY2 and on the other hand to groundthrough an admittance nY3. At point B1, a potential will be establishedequal to that appearing at each of the n points B which are directlyconnected .to point An. Further, point B1 is still connected to groundthrough an equivalent admitttance indicated in Fig. 4 and whose value isexplained by the preceding considerations. Point An is of course alsoconnected to ground through an admittance Y1.

If the admittances Y1, Y2, Y4 and Y5 are all constituted by purereactances, the source e of admittance Y is connected to the whole ofthe n loads Y3 by a purely reactive network. The latter will thusdissipate no energy except the -residual losses of the reactiveelements. 'I'hese losses represent however a secondary eiect andgenerally negligible, particularly in what concerns the condensers.Energy losses may also result from the absence of matching between thesource admittance Y0 and the input admittance of the network on the onehand, and between the combined load admittance nY3 and the outputadmittance of the network on the other hand.

However, it is known that a purely reactive network may be used tom-atch a resistive source to a resistive load without mismatch losses.In this respect one may for example usefully refer to the book ofEveritt entitled Communication Engineering and more particularly toChapter VII treating impedance transformations, pages 241, 246, 249 and2634267.

Briey, for a purely reactive 1r network interconnecting a resistivesource of conductance GA to -a resistive load of conductance GB andcomprising the admittances J'YA and jYB respectively in shunt on GA andGB, as well as the admittance jYc interconnecting the source to theload, the power in the load can be expressed by where e represents theof the source.

The `denominator of vthis yexpression comprising the sum of Ythree termswhich are prefect squares, and the first .term being constant, thecancelling of the two other terms corresponds to maximum energy beingsent into the load, i.e.

2GA 4 This corresponds to the reactive network matching the load to thesource. The cancelling conditions are:

These two conditions imply that one of the reactances must necessarilybe of a sign opposite to that of the other two at the frequency of thesource, and that the coupling should be sufficient, i.e.

' YczGAGB For .perfect matching, it may prove to be necessary addreactive elements of appropriate value and sign either between thesource and ground, or between the A points and ground, or else betweenthe B points and ground. Of course for the design of the matchingconditions one will take into account the reactive components of thesource admittance Y0 'as well as `those ofthe load admittance Y3. Theload Y3 comprises the admittance of the detecting device connectedbetween each B point and ground.

In cases where it is desiredy to use reactances of the same sign, it isto be noted that Relation 5 may be kept with a change of sign. In thiscase, the Relation 6 will still cancel the third term of the denominatorof 3, but the modied Relation 5 will produce a minimum Value for thesecond term, i.e.

The energy transmitted to the load will however always be lower than themaximum energy corresponding to perfect matching between the source andthe load.

As no part of the input energy is applied to detecting devices which donot characterise the input point, the equivalent network associating theinput point to the corresponding detecting points is purely reactive,and nally, a perfect impedance match between the source and the loads ispossible so that, one can obtain maximum energy from the source. Thismaximum energy will be entirely dissipated among the n detecting deviceswhich characterise the A point to which the A.C. energy source isapplied. Consequently, the detecting devices can be very simple and neednot require an individual amplification system for each B point such aslfor example an amplifying tube. A relay of the ordinary telephone typecan be connected between ground and the corresponding B point by meansof a rectifying system which may eventually be constituted by arectifier in series with the winding of the relay. As mentioned above,in parallel or in series with this device one can branch any reactivedipole useful to ensure a proper impedance match in order to draw theAmaximum of energy from the source. The use of an individual transformerforming part of each load ladmittance Ya is not in principle essentialbut it may prove useful to increase the admittance of the rectifyingsystem and the relay between each B point and ground. It may be usefulthat the equivalent admittance between each B point yand ground shouldbe high in order to ensure than any imperfection in the neutralisationof the couplings between B points should have the smallest possibleelfect. Indeed, if a certain non-zero admittance remains between the Bpoints represented by B1 in Fig. 4 `and the other (m-n) points B; thiswill correspond to a non-indicated admittance in Fig. 4 being derivedbetween the B1 point and ground. As this parasitic admittance will inany case be small, and if on the other hand the admittance between the Bpoint and ground is high, rthe parasitic shunt will be substantiallywithout effect. In principle, the value of the parasitic couplings isessentially determined -by secondary effects such as the manufacturingtolerance that one may give to the various components intervening in theRelation 2, the resistive losses of the elements assumed to be purelyreactive (essentially, the Q factor of the inductances) and theVariations of certain elements such yas Y1 following parasiticcapacitance variations.

Parasitic capacitance variations may happen for example when theidentification system of Fig. l is applied to ya telephone exchange. Inthis case, the A points will be constituted by a test conductor for eachof the telephone lines to be identified. This test wire has anequivalent parasitic capacitance to ground the value of which depends onthe length of the wire. Preferably, one will take care that thisadmittance Y1 should be small with respect to nY2. Then, as indicated byRelation 2 the differences between the equivalent parasitic capacitancestowards ground for each of the test wires may be neglected.

For each test wire, the equivalent admittance Y1 may also vary when thetest wire is extended by means of finders or selector switches whichwill be branched on the line. In this case, the equivalent admittance Y1may substantially increase. It is in view of such variations that onemay consider it useful to operate with a substantially 'high admittancebetween the B points and ground. One may also provide `a condenserconnecting each test wire (point A) to ground, the value of thiscondenser being chosen Well above the maximum value of the parasiticcapacitance which may appear between the test wire and ground. Thisentails however the use of an additional component per line to beidentified, and the value o-f this condenser may be rather large. Hence,it seems more economical to keep to a choice of the value of thecondenser forming the admittance Y2 which will be sufficiently high sothat the value nY2 will always be substantially higher than the ValueY1; and also, to increase the admittance between each B point andground, which may necessitate the use of a transformer if the admittanceof the rectifying system and of the relay is too low.

In so far as obtaining adequate energy in the useful loads is concerned,when variations of Y1 remain vadmitted, one will design the elements forappropriate extreme value of Y1, and in order to obtain an output energysubstantially equal in the two extreme cases.

Finally, a parasitic effect which may also have a certain importance isthe parasitic coupling between two A points. Indeed, if, in a telephonesystem the A points are constituted by test wires producing a certainparasitic capacity towards ground depending on the length of the wires,these will generally be grouped, whereby a parasitic capacitance betweenA points is also to be considered. In practice, this mutual capacitanceis however sufiiciently low to be treated as a secondary effect. It willsuffice to choose for the condenser forming Y2 a reasonably high valuewith respect to the mutual parasitic capacitance, in such a way thatY1-i-nY2 will remain sufficiently high with respect to this mutualcapacitance. The ratio between these last two values determines thatbetween the potential applied to the activated A point and that reachinganother A point by means of the mutual capacitance.

An identification network permitting identification of points by meansof m indicating devices having been described, we will now describe asimilar identification network using a coding system also of a generalscope but of a type different from the network shown in Fig. l. Inaccordance with this 'coding system, well known in itself,.p groups of mdetecting devices are provided in any among order to be able to identifyany among a plurality of mp points.

In accordance with this arrangement shown in Fig. 5,.

each of the mp points A will be connected to a point B in each of the pgroups of m points B. The symbolic representation system adopted in Fig.l has been kept in Fig. 5, and a single A point connected to a single Bpoint by means of an admittance Y2 has been shown. As previously, eachpoint A is connected'to ground by an admittance Y1 and each B point isconnected to ground by an admittance Y3, the latter essentiallycomprising the admittance of the detecting device. As indicated by themultipling arrow p, the COD1 network essentially corresponds to a groupof m points B, the other p-l groups being identical. As there are on thewhole pmp admittances Y2, and pm points B, each point B is foundconnected to v pmt points B to a unique point F which it itselfconnected to ground through an admittance Y5.

Although one may analyse the circuit shown in Pig. 5 in accordance withthe method used for the circuit shown in Fig. l, the reasoning becomessomewhat more complicated and it will be preferred to analyse thecircuit of Fig. 5 in accordance with another method. The latter willnecessitate ysomewhat longer explanations particularly for the generalcase envisaged, but it introduces complications which are only of analgebraic kind, and it permits easier establishment of an equivalentcircuit.

Considering the circuit of Fig. 5 in accordance with this last method,the symmetry of the circuit enables that the pm points B to be dividedinto two classes, a first class of p points B which will be directlyconnected to the point Ap to which the source shall be applied, and asecond class of p(m-l) points B which are not directly connected to thispoint A1,. The B points of the first class will all be at the s-amepotential yet to be determined. Likewise, the B points of the secondclass will also be at an identical potential yet to be 'determined anddifferent from the rst common potential.

The existence of two classes of B points having been noticed, a similardivision into classes of points will be made for the mp points A. Thelatter will be divided into p-l-l classes. A first class of A pointswill comprise a single point, the Ap activated point which isconnectedto the p points B of the first class. A second class of Apoints willl comprise all those which are connected to p-l points B ofthe first class, as well as to a point B of the second class. The numberof these A points pertaining to the second class is equal to p(m-l). Ina general manner, for the class of A points connected to x(p?x 0) pointsB pertaining to the first class; and consequently to p-x points Bpertaining to the second class, their number may be expressed by:

In the same way as for the B points, the A points of a same class willbe at a same potential and consequently a circuit equivalent to that ofFig. 5, when `an energy Vsource is applied to one of the A points, canbe established by taking into account only p-l-l points AA nected toground through admittances equal to pY1,I andaaiftee p(m-1)Y3. `On theother hand the B1 kpoint will be connected to the Ap point to which thesourcee ofthe admittance Y1, has `been connected, by means of an,admittance pY2. The point AX corresponds ,to .the common potential ofall points A forming the class of A points connected to x points B1.Consequently, it is on the other hand connected to p-x points B11, eachtime by means of an admittance Y2. Therefrom, yit results vthat theadmittances indicated in Fig. 6 as respectively connecting the points AXand B1 on the one hand, and Ax and B0 on the lother hand, have theindicated value. Further, all the A points of the gc class are connectedto ground, each through an admittance Y1, and the total admittanceindicated between point Ax and ground is thus that represented in Fig.6.

For each class of A points there exists therefore a T ,network betweenthe points B1, B and ground. l This equivalent network has beenindicated by ENWX for the A points of the x class, and the multiplingarrows marked by p and next to the points B1 and B11 thus indicate thaton the whole there are p networks of this type each time interconnectingthe points B1, B11 and ground. There exists of course a p-{-1)th networkformed by the single A point of the p class and solely connected betweenB1 and ground, but this network is already indicated separately on theleft of the figure since the.source e is applied to this point Ap.

it is therefore seen by considering the symmetry of the network of Fig.5 and by subdividing the various points into classes of points havingthe same potential, that an equivalent network corresponding to theapplication of the source to a particular point can be immediatelyestablished. It is to be noted that the network of Fig. 6 does notinclude the elements due to the BUN circuit. These elements will beshown separately in order not to complicate the figure. The BUN networkdoes not however affect in any way the symmetry of the circuit sinceeach of the B points is connected to a single point by means of anidentical admittance Y4.

The equivalent circuit of Fig. 6 will be simplified in the followingway. Each Ax point can be eliminated by a star-mesh transformation suchas shown in Fig. 7 which is a network which is wholly equivalent to thatof Fig. 6 and from which all the A points except the driving point Aphave disappeared.

Considering the equivalent network of Fig. 7, it is seen that theequivalent admittance interconnecting the B1 and B0 points is equal toLikewise, the equivalent admittances of the B1 and B1, points' towardsground will be respectively given by The expressions which precede arehowever in a somewhat complex form, but they will be appreciablysimpliied by evaluating the various summations, i.e. by eliminating thevariable x. These summations may be eliminated by using the binomialtheorem and can be eX- pressed by By dilerentiating (11) with respect tom, one obtains:

14 This .immediately vgives the value of ,the yswnination' appearing inthezExpressien 1130 which corresponds videntically to the summationobtained 'in the Identity 12.

By linearly combining the two Identities 11 and 12 the first beingmultiplied by p and the second by 1, one obtains and which gives thevalue of the summation appealing in the Expression 9, with thisdistinction that the summation of the Identity 13 is higher by p thanthat of EX- pression 9 since the first named summation comprises thevalue x=,p while lthe second stops at the value x=p-1.

The value of thesurnmation appearing in the Expression v8 remains to beobtained. A second different summation of the Identity 12 with respectto m will give:

=p p m-1 mrs-wlweQe-wym-1)-1 (le x: A linear combination-of theIdentities 11, 13, and 14 based on the identity:

x(p-x)=p2px(p*x)2 will finally give:

ptp-1 tm-ump-hxe-awww# 16) Consequently, the equivalent circuit of Fig.7 can in its turn be replaced by the substantially equivalent circuit ofFig. 8.

The equivalent circuit of the BUN network is represented in Fig. 9. Itcomprises only three star connected admit-tances and the F point can beeliminated in the usual manner to produce the equivalent circuit of Fig.10.

The complete equivalent circuit of the network of Fig. 5 will thus berepresented by the superposition of the circuits of Figs. 8 and 10. Inso far as the equivalent admittance between the .points B1 and B0 isconcerned, the lattercan be made equal to zero by satisfying therelation:

Then, the B11V point is Vcompletely decoupled from the B1 point and may.disappear from the equivalent circuit. When the Relation 20 issatisfied, the nal equivalent circuit is that represented in Fig. 11.This final equivalent circuit corresponds to the final equivaientnetwork which lhad been obtained in Fig. 4 for the network of Fig'. 1,and the considerations previously given in relation to the finalequivalent circuit of Fig. 4 are also applicable to that of Fig. l1.

Referring to the equivalent circuit of Fig. 8 which is that of thecircuit of Fig. 5, with the exception of the back-up neutralisingnetwork BUN, this circuit gives a very precise idea as to theconsidera-tions' given at the beginning of the description with respectto a system of the type described in the U.S. Patent No. 2,672,515. Ithas been said that the conditions giving a limited backup were inopposition with those permitting a good trans- 15 mission of the usefulsignal. By considering Fig. 8, the spurious back-up transmission can beexpressed by the ratio between the admittance interconnecting B1 and B1,to the sum of this admittance and of the admittance between B11 andground. If one assumes that the admittance Y3 is very low, which is`logical since the input of each detector, in view of the smalltransmission of the useful energy will be general drive the `grid of anamplifying tube, the inverse ratio expressing the los-s may be written:

On the other hand, the useful transmission can be expressed by the ratiobetween the admittance interconnecting Ap to B1 and the sum of thisadmittance to that. interconnecting B1 Vto ground, if one neglects theadmittance interconnecting B1 and B11 for the calculation of the usefultransmission. Therefore, for the inverse ratio expressing the loss oneobtains:

Y1 The value of the Expression 22 giving the voltage transmission lossestowards the detectors which must be operated, to make it as small aspossible, it is of interest to choose Y2` much larger than Y1. However,as the val-ue of the Expression 2l which represents this spurioustransmission loss must be as large as possible, this indicates Y1 mustbe much larger than Y2. Thus it is clear that the two condi-tions arediametrica'lly opposed. The solution proposed in the U.S. Patent No.2,672,515 consists in choosing a ratio between Y1 and Y2 which is oflthe order of 100 which, for m1=10 land p=4 (10,000 line exchange) givesa value of 334 to the Expression 2l. With respect to the useful signal,the parasitic signal brought to the detectors which must not beactivated thus sulers a yloss of about 50 decibels. But on the otherhand, the same ratio between Y1 and Y2 with the same numerical values ofmi and p gives to theExpression 22 a value of 954. This means that withrespect to the input signal, the useful signal which must operate theparticular combination of detectors will be subjected to a loss ofnearly 60 decibels. This clearly expresses the necessity to use in thatcase complex detectors each comprising an appreciable number of tubesand associated circuits. Indeed, the very small available energy must beconsiderably amplified in yorder to permit the nal operation of a relayterminating the detector. On the other hand, the fact that the usefulsignal at the input of a detector is at a very low level precaution toobtain a sufficient immunity with respect to spurious signals.

`'It will be remarked that the coding system of Fig. 5 has the advantagethat it is readily adaptable to decimal numbering. Indeed, if onedesires to use the system shown in Fig. 5 to identify telephone lines ofa 10,000 line exchange, this will correspond to m= and p=4, and one willhave four series of 10 detecting devices permitting the identificationof the number of a line by its four conventional decimal digits for thethousands, the hundreds, the tens and the units. If one used the codingsystem of Fig. l for a 10,000 line exchange, the detecting devices wouldnumber 16 only, but the combi-nation of 8 relays which would be operatedto characterise the line to be identified would have no relation to thedecimal system. Of course, although the two coding systems reespectively represented in Figs. 1 and 5 have a general scope, they arenot the only ones which can be used and they are solely given asrepresentative examples. For` insta-nce, one could combine the principleof the 'operation of a constant number of detectors in a group (Fig. 1)with that of the use of several distinct groups. In other words, thearrangement could be analogous to that of Fig. 5, but in each group, ofdetectors one would operate not one but a predetermined number greaterthan one. To x ideas, in a case of a 10,000 line exchange, one could forexample couple each of the 10,000 lines to be identiiedto detecting'devices, each time with the help of 8 elements, 2 elements going towards2 detectors in a first group of 5, 2 other elements going towardsanother combination of 2 detectors out of a second group of 5, and soon, which would indeed produce possible combinations of 8 activateddetectors out of the 20 detectors used for the system. A system of thistype could be provided with a back-up neutralising circuit such as shownin the Fig. 5 and the circuit analysis could be made in the mannerindicated for the circuit of Fig. 5.

It will have been observed that this last coding system as well as thoseof Fig. l and of Fig. 5, are essentially symmetrical, each group ofdetectors possessing the same number of detectors, and in each group acombination of a number of detectors is operated which number is thesame for each of the groups, and the same of course for each of thecombinations characterising a particular line. This is not essential topermit the application of a backup neutralising network. One may wellconsider two or several groups which do not comprise the same number ofdetectors. By way of example, the design of a neutralising network inthe case of non-identical groups will be studied.

i Fig. 12 represents an arrangement of this type using a rst group of mdetectors and a second group of m detectors, m' being not essentiallyequal to m. Each of the input A points to be identied is on the one handconnected to a B point out of the group of m points B, and on the otherhand to a point B out of the group of m' points B'. Hence, the systemwill permit the identification of an A point among mm' points A. Inorder to simplify the description, it will be assumed that theadmittance of each A point with respect to ground can be considered asnil. As shown by Fig. l2, the A point represented is connected to the Bpoint shown, by the admittance Y2 and to the B point shown, by anadmittance of the same value. As indicated by the multipling arrowsadjacent to the points B and B', each of those is respectively connectedto m and m points A.

As previously, each of the points B and B is connected in the BUN`neutralising network to a single point F, each time by means of anadmittance of value Y.1. Again to simplify the description, it will beassumed that the admittance of the point F with respect to ground can beconsidered as nil. Further, in View of the asymmetry between the groupsof B points, the BUN neutralising network comprises two other singlepoints G and G. The point G is connected to each of the m points B by anindividual admittance Y6 while the point G is connected to each of thepoints B by means of an individual admittance Ys. The admittance whichcould connect each of the points G and G to ground will not be takeninto consideration, again with regard to simplification.

Considering first of all the circuit of Fig. 12 with the exception ofthe BUN network, its equivalent circuit may be established exactly inthe same way as the equivalent circuit of Fig. 6 was established withrespect to the circuit of Fig. 5. For the circuit of Fig. l2, it willhowever be necessary to consider four classes of points B and B', thepoints of the vsame class having the same potential. Thus, there will bea point B forming a first class, which is that directly connected tothepoint A11 to which the identification signal is applied. Likewise, therewill be a single point B forming a second class and which is thatdirectly connected to the point A11. The third and fourth classes of Bpoints will respectively comprise the (rn-1) points B which are notdirectly connected to the point A11 and the (m-1) points B' which arealso not directly connected to the point A11. If one calls B1, B1, B0and B1, the points of the equivalent net- 13. The point A10 has apotential corresponding 'with' that of the m-l points A which are on theone hand connected to the point B1 and on the other hand to the.

point Bo. Likewise, the point A01 has a potential corresponding to thatof (m-l) points A connected on the one hand to the point B1 and on theother hand to the point B0. sponding to that of the (m-l) (m"-1)points'A whichare on the one hand connected to a point B and Von theother hand to a point Bo. Thus, the circuit of Fig. 13 shows` that thereexist undesirable couplings between the vpoints B1 and B'o on the onehand, and B1 and B0 on the other hand. The BUN network will permit theirelimination.

The equivalent circuit of the BUN network is shown in Fig. 14.r It willnot necessitate particular explanations, the admittance values indicatedbetween thepoints B1, B0, B1, B1) and F, G and G being readily deducedfrom the'BUN network shown in Fig. 12.

The complete equivalent network for Fig. 12 is thus obtained bysuper-posing the network of [Figg14 to that'of Fig. 13. By consideringthis complete network, it is clear that the points A10, A01, A01, on theone hand, and the points F, G and G on the other hand, are not ofinterest insofar as the determination of the conditions permitting theneutralisation of the back-up couplings is concerned. The points A10,A01, A00 are eliminated in an obvious way and similarly for the points:G and G', while the point F will be eliminated by a s-tar-polygontransformation. In particular, the network of Fig. 14 will produce anequivalent admittance between the points B1 and B'o which becomes addedto that shown in the circuit of Fig. 13. The same is true for theApoints B1 and B11. Without diiculty, it will, be ascertained that thereexists a common condition permitting to make both the equivalentadmittances between the points B1 and B1, on the one hand, and thepoints B1 and B0 on the other hand,

vequal to' zero. This condition is expressed by Y4=mm Y2 23) The networkhaving the point v1:" as central point thus permits by a judiciouschoice for thevalue of the admittance Y1 the elimination of any couplingbetween the points B1 and B1, and between the points B1 and yB11.However, this star network introduces a coupling Ybetween the points B1and B0 on the one hand, and the points B1 and Bo onthe other hand.Without the BUN network, the coding network does not introduce such=couplings. The purpose of the star networksindividual to each of thetwo groups of points B is therefore to neu- 'tralise these undesirablecouplings introduced by thek first part of the neutralising network.Without diiiiculty one :will establish that the condition permitting .toobtain a ;zero equivalent admittance between the points B1 and In ananalogous manner, the condition permitting to obtain an equivalentadmittance equal to zero between the points B1 and B1, is

l l YI.: m ,Yn (25) Finally, the point Awhas apotential corre! 18ditions expressed by the Relations 23, 24 and 25 are observed, since thepoints B0 and Bo are then completely decoupled with respect to thesignal source. Insofar as the equivalent admittance between the pointsB1 and B1 is concerned, it is also not necessary to take it intoconsideration when the above mentioned relations are satised, since inthat case Fig. 13 clearly indicates that the points B1 and B1 are at thesame potential.

In joining these two points, the complete equivalent circuit when theabove relations are satisfied therefore corresponds to the circuit shownin Fig. 15. The equivalentl circuit is thus reduced to a simpleadmittance of value 2Y2, which may berentirely reactive, connecting thesource to thetwo detectors which must be activated ,to characterise theA point to which the source is applied. If one had considered ltheadmittancesy of the various points with respect .to ground, one would ofcourse have obtained a quadripole connecting the source to the detectorsto be activated.

Having shown by this example that the neutralisation may also be appliedto non-symmetrical arrangements, it is4 also interesting to consider thecase where the numberfof elements forming a group may be modiiied fromtime to time. The case may arise for instance in the identification of'lines in a telephone exchangerwhen'the numberY of lines Yin theexchange increases. For instance, one ymay initially provide for 400lines the identification of which will be made with the help of threegroups of detectors, the rst and the second groups each comprising 10detectors anda third group only 4. If the number of lines increases',one must increase the number of detectors in the third group which mayfor instance become 6 if one reaches a 600 line exchange. The simpleexample of Fig. 12 and thecorresponding Relations 23, 24 and 25 indicatethat if m Afor instance is modiiied, Y1 and Y must be modified. If theadmittances Y2 are constituted by condensers, it is evident that theadmittances Y6 and YB shall also be condensers, but that the admittanceconstituting Y4 shall have to be an inductance. Consequently, when m ismodiiied, one will have to change mf' condensers and m-i-m inductances.If one considers that it is not desirable to have for each change invalue'of vm", to change m-i-m inductances which are in general morecostly elements than the condensers, the BUN neutralising network ofFig. 12 Vcan be slightly modiiied in' order to avoid the replacement ofall the m-l-m inductances each time that m' is changed. .'Fig. Al-6represents the modied BUN network. AThe network `is lanalogous to thatshown in Fig. l2 except for the fact that the point F has beenduplicated into two points F andv F interconnected by -a singleadmittance Y4o- Theequivalent circuit of the BUN network of Fig. 16shall therefore be established in the manner represented in Fig. 17. As`the equivalent circuit of Fig. 14 was grafted on that of Fig. 13, thatof Fig. 17 will be grafted on that of Fig. 13. t Y

v The common condition permitting to render bot-h the vequivalentadmittance between the points -B1 and B1, and that between the points B1and B1, nil, can now be expressed by which therefore replaces theRelation v23. In order to make the value Y4, which corresponds to m-l-melements, independent of the value of tm', one will choose whichcorresponds to Relation 23 in the case where m L. On the Yother hand,the respective conditions permitting to annul the equivalentadrnittances between the points B1 and B0 on the one hand, and thepoints B'; and B'o on the other hand become instead of the Relations 24and 25.- v

The Conditions 27, 28, 29 and 30 show that only the single admittanceY@and the m admittances Y6 are function of m'. In the case where Y2 isconstituted byV a condenser, theonly inductance which must -be modifiedwhen m varies .(m m l) is that-corresponding to the admittance Y40interconnecting the points F and F.

The various examples `of coding systems whichhave already been explainedabove and for which it has been shown that it was each time possible` tocompletely neutralise the undesirable 'back-up couplings, and tousesubstantially all theV energy from Vthe source to operate the detectorscorresponding to the point to be identified, clearly indicate Athatthere exist no particular restrictions asto the application of aneutralising network. However, the examples shown heretofore have allbeen of a type in which the connection between a ypoint lA to beidentified and a corresponding detecting point B'is'effective by meansof a single dipole directlyinterconnecting these two points. But, it isclear that in certaincases a codingeftected in atleast-two stagesofinterconnections may' be desirable, Indeed,` if one considers forinstance the case of a 10,000 lineexchange and a one-stage coding systemtowards four groups of 10 detectors, each line must at least be providedwith four elements such as condensers of which one goes 'to a detectorin each ofthe four groups of detectors. On the other hand, if the cddingnetwork is established 'in at least two stages, one shall be lable tolimit the'number of elements connected Vto each line to two only. Onemay use vtwo series of 100 intermediate points between the 10,000 pointsA to be identied and the 40 points B to which the 40 detectors areconnected. Each point A'will be connected to an intermediate point ineach group of '100' by a dipole 'constituted for example by a condenser.4In their turn, each ofthe 100 intermediate points of a group of 100points will' be connected to' av detectorfout of a rst group of l0 andto a detector out lof a4 second group of 10, each-time Vby a singledipole which can again be constituted by a 'condenser.v For such'anarrangement, the-*numberof elements in the coding networkfse'rving tointerconnect the points Alto' the points B is therefore equal'to20,000i-400=20,400 Practically speaking, the number of elements per linehas thus been reduced to 2 `instead vof 4. 'An arrangement of this typeis infact envisaged in lthe U.S. Patent No. 2,672,515.

Fig. l8 shows an identification network comprising two stages andprovided with Van adequate'neutralising circuit. It is considered thatthere are m4 points A of which each is vconnected byv means of anadmittance Y21 to two' points D, o'ne among a first group of m2 points Dandthe other among a second group of m2 points D. As indicated by themultipling arrow ac'lja'cent to the poitsD shown, each of those isdirectly coupled to m2 points A. ,In their turn,'each of the pointsD isconnected 'by means of an admittance Y22 on the one hand to' a point Boutfof a rst group of m; and "onthe other hand point B out of anothergroup of m points B. As indicated by the multipling arrow adjacent tothe points B shown, each of these is'thus connectedeach time by meansof' an admittance Y22 to' m points'D. 'In' the case where 'm'=10, the10,000 lines will thus be identified by the points B on the basis of 4decimal digits and'the signals at the D points will correspond to anidentification on the one hand of the thousands digits associated to1he.hundredsdigits,--fand`* on the other hand 'ofthe tens digitsassociated to ltheV be established by the method of-subdividing thevarious points' into'classes ofpoints being at the same potential.

One will consider first of all the 4 m points B. Due tol the symmetryofthe circuit, .these 4 m points B are divided into a first class of4points B1 directly connected to the point -A5 to which the source isapplied by means of an admittance Ygzin'series with an admittance Y21,and a second class of 4.(m-1) points B0 which are not directly'connected toth'e source in this manner.

lConsidering-now the 2 m? zpoints D, these can be subdivided=-into threeclasses. The first classfcomprises the 2 points' D2'which aredirectlyconnected each to 2 points B1 by means of an admittance Y22. Thesecond class of points D comprises" 4"(m-l) points D1 which are eachconnected 'by 'an' admittance Yzz'on the one hand to a point B1'tandsonfthe other hand to a point B0. Finally, the third 'class of-Dpoints vcomprises 2 (m-l)2'points Dowhichare directly connected by anadmittance Y to 2 points B0.

Similarly, the m4 points -A shall be classified into six classes linaccordance with the combinations of points D2, D1 and D0 to which theyare associated. Point A5 will'be that directly connected by anadmittance Yzl to thetWo-points D2. Evidently, it corresponds to thepoint at which the energy source is applied. The table belowindicatesthe subdivision o'f the points A into 6 classes in accordancewith theirdirectconnections to the points D, as well as thenumber-.ofpoints in each class.

y As -DZ'D2 1 A4 DZ'D1 4011-1) vA., D213o zon-n2 A2 v D1D1 '4(m-1)2 A1'DID() A0 DOD() The subdivision of the` various points B, D and A intoclasses'jof pointshaving the same potential inside eachclassfhavingfbeen defined, kthe equivalent circuit of the networkmfFigjllS canbe drawn. As previously, the eguivalentcircuitk"corresponding to the coding network will beV shown separately,theequivalent circuit of the BUN neutralising networkbeing shown apartin `order not to complicate' the figure. it will Vbe noted that Fig. lSdoes not'indicate admittances towards ground neither for the`A;p`oints,"nor Vforjthe'D points, nor for the point F, no rforthe'rpoint G. These have been omitted with a view y"o' r`simpliiication'a'nd theyv are not essential to obtain the neutralisationof theundesirable Vback-up couplings. i In Fig. "l9-,-all 'theadmittance values interconnecting 'the various "points have beenindicated and they cor-` respond in a manner which will easily beverified, to the various numbers of points forming each class. By wayof'example, as there are 2 '('m-l')2 points A3 and as thesevare definedas being connected to a point D2 and a "poin'tDfjfA thefadmittance's-between Aa'and D2, and betfw'een'f'Ag'f and Do-mustthusbe'equal to 2 {frz-U23@ as indicated. It is immediatelyseen-thatthe'-adniittances respectively interconnecting the points DOand A0 as weil las `D1 and A2 need-notrbe'taken into consideration.

Theyrshould only be if one had introduced the admittance`Y1*in"tereorine'ct-ingeach `A point to ground.

f By-:eliminatingthepoints A0, A2 as -well aslpoints A4, 'A3' andAJ, oneobtains -the'cir'c'uit lof Fig, 20 Vwhich is .substantiallyeq-invalent1t@the circuit of F1919.

"The equivalent'circuitof the BUN neutralising `net- .work'wil'lnow' bensidered The iirst part of this equivalent 'circuiti corresponding tovthe BUN 'networkis #represented-iin -Fig. 21. --As shown'v.byv--this-liigine; itis uit l By a star-triangle transformation, thecircuit of Fig. 2l

becomes as indicated in Fig. 22. This circuit must therefore besuperposed on the circuit of Fig'. 20. If 'theyrelation T` l '5 im2Y21+2Y41 =0 l f 'Y Y(31) is satisfied, `Athe4 superposition uof theequivalent network of Fig. 22 upon Ythe network' of,l-igfZQQwillVsimultaneously entail the cancellation of the admittance interconnectingthe points D2, D1.andD in pairs. Y

' Consequently, when the Relation 3*'lv'yis satisfied, the superpositionof the networkspof Figs. 20 and 22 produces the equivalent networkofFigL23. `This last iigure shows that the 'points B1 and ,Bo are nolonger coupled except' by two admittances in series by means of pointD1. On the other hand, it still remainsto superpose the equivalentnetwork vconstituted bythe admittances Y@ of the network BUN whichinterconnects each of the 4 m points B to the single points F. Thisequivalent network is shown in Fig. 24. Y

' By superposing the circuit of Fig. 24 to that of Fig. 23, one readily'obtains the iinal equivalent network corresponding to the whole of thecircuit shown in Fig. 18. If one satisfies the relation A, .I A `theequivalent admittance remaining `between the points Bland B0 iscancelled. Hence, thepOint'Bo is completely dissociated from the pointB1 provided that the Relations 31 and 32 are satisfied. No fraction ofthe input energy is derived towards the 4 (m-l) detectors which do notcorrespond to the A point to which the energy source is applied. j

The iinal equivalent network, after elimination of the point D2, becomesthat shown in Fig. 25. Source e is simply connected to the 4 detectorscorresponding 4to point A5 by means of a series admittance. If Y21 andY22 are'constituted by condensers, this series. admittance will becapacitive. 3l and 32 indicate that the admittances Y41and'Y42 are bothconstituted by inductances. y

Thev circuit of Fig. 18 establishes that the neutralisation is equallyapplicable in the case of complex4 coding networks where the'coding isperformed in stages. y

We will now consider-'in a more detailed manner, the natureof theelements `which.may be used to,form the identifying networks describedabove, including `the neutralising network incorporated in theidentification circuit. If one desires to apply thev arrangementsdescribed to Ithe identification of telephone lines, it will in generalbe desirable that the admittance such as Y2 represented in Fig, 5 shouldbe constituted by a condenser. Indeed, in this way the A points will beinsulated from one another for direct current. y a test wire of thetelephone line as input point to the identification network, while thistest wire is kept available for any other D C. signalling. For instance,it will be possible to reserve the test wire used for call metering andlwhich is thus connected to the subscribers message register. Further,the use of condensers for the coding network between the input pointsand the output pointsV possesses lthe advantage that they do notdissipate energy.

Hence, the series elements of the back-up neutralising network and whichare connected to the output points may be constituted by inductances.Such a neutralising network is shown in Fig. 26.

In Fig. 26 each of the output points B is connected Vto a single point Fby means of an inductance L4 which thus corresponds to the admittancesY4 represented'. for example in Figs. 1 and 5. On the other hand, the Fpoint is connected to `ground by means of a condenser C5 which thuscorresponds to the admittance Y5 for ex-v On the other hand, theRelations' This permits the use of ,ample inV Figs.- 1 and 5. Theinductances L5, and the 75 network could for example be that of Fig. 1or that ofA Fig. 5 or any other suitable network. In a general manner itwill be assumedthat the coding network is such that when the whole ofthe networkv isv activated by the' application of an A.C. energy `sourceto one of the A1 input points, the B outputpoints are divided into twovclasses of equipotentialpoints: the iirst class comprising the B outputpoints which are activated and the second class comprisingv all theother B points. As indicated by the multipling arrow adjacent to pointF, a total of x (y-l-l)- points B is assumed, of which x can becomesimultaneously activated,while.the remaining xy points are not. On thisbasis, onev can thus represent the equivalent circuit of theneutralising network of'Fig. 26.

' Fig. 27 represents this equivalent network to which a condenser C hasbeen added between the points B1 and B0 representing respectively the xactivated output points B and-the xy output points B which are notactivated. This condenser C may thus for example correspond tot theadmittance which is shown in Fig. 8 as interconnecting the points B1 andB0. The condenser C5 is` not an essential element of the neutralisingnetwork. Generally, one may rather assume that it represents a parasiticcapacitancel between the point F and ground. In particular, the lattermay result from the concentration of the various conductorsinterconnecting the F point tothe x (y-i-l) points B. In order tosimplify the. analysis, one may assume that this capacitance C5 may beneglected either because it is of small value or because it will havebeen neutralised by placing in parallel with it an inductanceinterconnecting point F to ground and of such value that itforms ananti-resonant circuit with the capacitance C5. Of course, there will beno particular difficulty in taking into account the value of thecapacitance C5, and moreover, the dipole interconnecting point F toground may in fact be complex in order to have a particular ad' Y W21L40Ltg: l

must be satisfied in order to eliminate the equivalent coupling betweenthe points B1 and B0. It is to be remarked that if one takes intoaccount the capacitance C5, two distinct angular frequencies appear assusceptible of permitting the obtaining of zero equivalent admittancesbetween the points B1 and B0. However, such an arrangement will alsoshow that there exists a frequency for ywhich the equivalent admittancebet-Ween the points B1 and B0 becomes infinite which is evidently theopposite to lthe desired' result. One will therefore have' tosufliciently differentiate between the frequencies for'which theequivalent admittance between the points B1 and B0 is cancelled and thatfor which it becomes infinite.

The possibility to obtain a zero equivalent admittance between thepoints B1 and B0 for more than one frequency, is interesting since itenvisages theuse of the network at two distinct frequencies, e.g. 20kc./s. on the one hand and 30 kc./s. on the other hand. This wouldpermit effecting as many simultaneous identifications as there aredistinct frequencies for which the equivalent admittance between theactivated output points and the others becomesnil. It will onlybenecessary to provide as many detecting devices peroutput point as thereare sources Winding ofa transformer T infserieswith an'n'djuQtanQe; 10;;

L31and a condenser C31. The series combinationformed by theV inductanceL31 and'thecondenser.y C31 kisgin.parallel:Y with condenser C33. formerT is in series with the Winding of a relay'fRrf andy a rectifier S. Wehave assumed here the use oftfai transf. former inorder to be ablev toadapt the series impedance formed by,v the relay-and the rectifier toanadmittance.. value which is relatively high and which, aspreviously Amentioned, offers the advantage to be guarded= againstV secondaryeifects such as the Q factor of the inductances` L4 which might leave asmall admittancebetweenthe points B1 and B0. On the other hand, the Bpointsis also connected to ground' through a parallel circuit identicalvto the previous one with respect toits structure, except?v for the useof an inductance L33 in the place 'of a con. denser C33. The rectifyingsystem is shownherein its. simplest form as a half-Way rectifier. Onemay Yof course use any rectifying system able to providethe.y bestoperating conditions for the relays.

The combination L31, C31, C33 is. able to produce a series resonance ata first frequency, and also a parallel resonance at a second frequencywhich must necessarily be higher than the first. On the other hand, 'thecircuit L32, C32, L33 is able to produce a parallel resonance at thistirst frequency and a series resonance at this second frequency higherthan the iirst. This is well known, and these conditions will besatisfied by the relation Hence, for the firstfrequency, the seconddetecting circuit comprising the relay Rr will .be completely decoupledfrom the B point `and will receive substantially no energy. In ananalogous manner, for the second frequency, the rst detector circuitconnected to the B-point and cornprising the relay Rr will becompletelydecoupledand will receive no energy.

In order to obtain a. good discrimination between .the two frequenciesable to be used on any other frequency for which an iniinite admittancemaybe obtained between the points B1 and B0, the neutralising networkmay be embodied as shown in Fig. 29.

ln the circuit of Fig. 29, the admittance which may connect the singlepoint VF to ground will not be taken into account and it will be assumedto be zero. It is seen that'each B point is connected to the F point bymeans of la complex dipole comprising the inductance L42 in series 'withthe parallel combination of the inductance L41 and of the condenser C41.Again, it will be assumed that there are x points B activated and xywhich are not.

The circuitof Fig. thus represents the equivalent circuit 'ofrFi'g 29such asv it is offered between the points B1 and'Bo, the condenser Cvbetween these two points again representing the equivalent admittancesolely resulting'from the coding network between the input A points andtheoutput B points.

Without difficulty it will be seen that the circuit of Fig. 3l issubstantially equivalent to that ofV Fig. 30. The circuit ofFigfBl showsitself as a purely reactive, complex dipole between the points B1 andB1, and comprising two inductances and/two condensers. Thisdipole willvbe recognized as being one of those structures com prising twoinductances and two condensers and whichl may have an identicalcharacteristic, and such that thereexist twmparallel resonancelfrequencies sand Ione` series The t' secondary winding' off transr:

resonance frequency.

K. S. Johnson entitled Transmission Circuits for TelephonieCommunication (1924).2V and particularly to pages 271 and. 300. One willchoose valuesfor' the elements,` L41, L42 `and C .11such that onelobtains, two parallell Vresonance frequencies for the circuit of'Fig.3l', correspending respectively tothe first, and second frequenciesmentioned. inrelation to the circuit of Fig. 28. One will choosethem`alsoin ordertor obtain av good discrimination with respecttogthe seriesresonance frequency for the.`

circuit of Fig. 31.1 -Y Y .Until new, purely reactive elements havesolely. been considered for; constituting the back-up neutralisingnetwork. Whileit isin principle desirable to use solely reactiveelements both for the coding network and for. the neutralising network,inorder toavoid energy losses, this is notabsolutely essential. in what.concerns theelimination of the eiective admittance betweenthe activatedoutput points andthe others. One may lfor examplekeep condensers forconstituting the coding network but use only condensers and resistancesto constitute -the neutralising, network, the inductances beingthusavoided.. In a general manner, one will endeavor to. constituteA anIRC neutralising network of low pass structure type between the poi-ntsB1 and Bo, in parallel with an RC high-pass type structure, due accountbeing made of the equivalent condenser between these two points and'.resulting from the coding network.

Such structuresmay offer the property of providinga Zero transfer`admittance for' 'f1-particular frequency. It is-to be remarked, however,that in opposition to a purely reactivefnetwork, the equivalent couplingnetwork between thepoints B1' and B0 shall necessarily have to beanunbalanced quadripole using ground, or a balanced1 quadripole when theground return is not used, but not a simple equivalentdipole between thepoints B1 and B0.

The best knownv RC structure of this type is the twin T. .It is howevernot unique and there exist manyothers. in particular, in the articlepublished in the August' 1948 issue of Proceedings of the Instituteof'Radio Engineers page 882, and entitled Bridged reactance-resistancenetworks, G. R. Harris has described various quadripoles eachcomprising. 3fcondensers Aand 3 resistancesy andlable to provide a zeroltransfer admittance at a particular frequency.

Fig. 32 represents a BUN neutralizing network-r of the of the RC-type.Each ofthepoints B is connected to'a single point F bymeans of' aT-network comprising-av resistance R4 connected to the point B on theone handy and on theother hand tothe point F byV means of the re'-sistance The junction .point of these two resistances is connected toground. by means of the condenser C5. Again assuming that x points B areactivated and that the remaining xy points are not, one may representthe equivalent circuit of the BUN network.

Fig. 33 represents this equivalent circuit to which, as previously, theequivalent condenser. interconnecting the points B1 and B1, andresulting from the coding network has been added. The circuitessentially comprises three equivalent resistances and three equivalentcondensers. It will be recognized as one described by Harris in theabove mentioned article. By star-delta transforma.- tions, one willobtain the equivalent admittance between thepoints B1 and Boand thelatter may be cancelledat a particular frequency depending on the valueof the various elements, `and provided also that a particular relationexists between those. This may be expressed by HOnthe...otherhand, if W1again represents the patricu- Such reactive dipoles are well known andone may for example refer toy theyboolqofj

